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Lateral Ramps in the Folded Appalachians and in 

Overthrust Belts Worldwide—A Fundamental 

Element of Thrust-Belt Architecture

By Howard A. Pohn

U.S. Department of the Interior

U.S. Geological Survey

U.S. GEOLOGICAL SURVEY BULLETIN 2163

Lateral ramps in the subsurface can be detected from 

changes in surface folds. Such changes occur in

fold-and-thrust belts worldwide and probably 

indicate lateral ramps.
Pohn, Howard A.

 Lateral ramps in the folded Appalachians and in overthrust belts worldwide—

 A fundamental element of thrust-belt architecture / by Howard A. Pohn.

 p. cm. — (U.S. Geological Survey bulletin ; 2163)

Includes bibliographical references.

Supt. of Docs. no.: I19.3:2163

1. Thrust faults (Geology)—Appalachian Mountains. I. Title. II. Series.

QE75.B9 no. 2163

[QE606.5.I5]

557.3 s—dc21 98-48485

[551.8′72′0974] CIP

Library of Congress Cataloging-in-Publication Data

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 2000

Published in the Eastern Region, Reston, Va.

Manuscript approved for publication August 10, 1998

Any use of trade, product, or firm names in this publication is for descriptive purposes only and

does not imply endorsement by the U.S. Government

For sale by

U.S. Geological Survey

Information Services

Box 25286, Federal Center

Denver, CO 80225

or call the toll-free telephone number

1-888-ASK-USGS
CONTENTS

Abstract........................................................................................................................... 1

Introduction .................................................................................................................... 1

Purpose and Scope.................................................................................................. 2

Acknowledgments .................................................................................................. 2

Geometric Considerations of Lateral Ramps.................................................................. 4

Overview ................................................................................................................ 4

The Basic Model..................................................................................................... 5

Field Examples and Supporting Data ............................................................................. 7

Susquehanna Lateral Ramp .................................................................................... 7

Stratigraphic and Structural Setting................................................................ 7

Seismic Evidence............................................................................................ 10

Dip-Line Section..................................................................................... 10

Strike-Line Section................................................................................. 11

Pennsylvania-Maryland-West Virginia Lateral Ramp ........................................... 11

Mathias Lateral Ramp ............................................................................................ 11

Surface Evidence ............................................................................................ 11

Seismic Evidence............................................................................................ 11

Highland County Lateral Ramp.............................................................................. 19

Surface and Geophysical Evidence ................................................................ 19

Seismic Evidence............................................................................................ 19

Igneous Intrusions Associated with Lateral Ramps ............................................... 19

Additional Lateral Ramps in the Central Appalachians ................................................. 19

Wilkes-Barre Lateral Ramp.................................................................................... 19

Seven Mountains Lateral Ramp ............................................................................. 26

Tyrone-Mount Union Lateral Ramp....................................................................... 27

Bedford Lateral Ramp ............................................................................................ 27

Lexington Lateral Ramp......................................................................................... 27

Roanoke Lateral Ramp ........................................................................................... 27

Fincastle Conglomerate: An Indicator of the Roanoke Lateral Ramp .......... 30

Changes in Unit Thickness and Lithologies................................................... 31

Coarse-Pebble Conglomerates Related to Other Ramps ........................................ 31

Lateral Ramps in the Southern Appalachians................................................................. 31

Lineament Swarms of Eastern Tennessee and Western North Carolina ........................ 37

Lateral Ramps and Tectonic Windows........................................................................... 37

Lateral Ramps and Frequency of Disturbed Zones ........................................................ 46

Lateral Ramps and Possible Basement Faulting............................................................. 49

Movement Along Lateral Ramps Through Time ........................................................... 50

Observed Association of Lateral Ramps and Giant Landslides ..................................... 51

Possible Relationship of Lateral Ramps to Offshore Transform Faults......................... 51

The Presence of Lateral Ramps Worldwide ................................................................... 52

Lateral Ramps in the Brooks Range, Alaska.......................................................... 54

Lateral Ramps in Papua New Guinea..................................................................... 57

Lateral Ramps in China .......................................................................................... 57

Conclusions .................................................................................................................... 57

References Cited............................................................................................................. 62

III
IV CONTENTS

FIGURES

1. Block diagrams of lateral ramps ....................................................................................................................... 2

2. Side-looking airborne radar (SLAR) uncontrolled image of the southern Appalachians showing the 

location of the Jacksboro fault and the Pine Mountain thrust system in western Tennessee............................ 3

3. Simplified block diagrams of lateral ramps showing four basic geometric configurations ............................. 6

4. Illustration of the scale of folds, faults, and ramps........................................................................................... 8

5 Sketch showing the relationship of a décollement to primary, secondary, and tertiary faults.......................... 10

6. Dip-line seismic-reflection profile in northeastern West Virginia showing the relationship of 

tectonic ramps to basement block faults ........................................................................................................... 12

7. Block diagram showing relationship between basement block faults and frontal ramps and between 

basement cross-strike faults and lateral ramps.................................................................................................. 14

8. Block diagrams showing relationship of fault refraction to steepness of fault scarp ....................................... 14

9. SLAR image of the southern part of the Williamsport 1¡×2° quadrangle and the northern part of the 

Harrisburg 1¡×2° quadrangle, central Pennsylvania......................................................................................... 15

10. Block diagram showing relationship of basement faulting, change in décollement level, and change 

in fold wavelength to the east and west of the Susquehanna River, central Pennsylvania............................... 16

11. Geologic map of central Pennsylvania showing approximate locations of the seismic-reflection profiles ..... 17

12. Stratigraphic section penetrated by the Amoco No. 1 Wilhour Gas Unit well from the

Shade Mountain area of central Pennsylvania.................................................................................................. 18

13. Dip-line seismic-reflection profile across the Shade Mountain anticline......................................................... 20

14. Dip-line seismic-reflection profile across the Milton and Montour anticlines................................................. 22

15. Strike-line seismic-reflection profile parallel to the regional structure but perpendicular to the

Susquehanna lateral ramp ................................................................................................................................. 24

16. SLAR image of part of the Cumberland 1¡×2° quadrangle showing the narrowing of folds to the 

south across the Pennsylvania-Maryland-West Virginia lateral ramp .............................................................. 26

17. Block diagram showing the eastward shift of the Appalachian structural front south of the 

Pennsylvania-Maryland-West Virginia lateral ramp......................................................................................... 27

18. Strike-line seismic-reflection profile of the Mathias lateral ramp in West Virginia......................................... 28

19. Sketch of a roadcut 0.6 mi north of Mathias, W. Va., along West Virgina Route 259, showing 

cross section of an up-to-the-north lateral ramp ............................................................................................... 30

20. SLAR images of parts of the Cumberland and Charlottesville 1¡×2° quadrangles showing the 

boundary lines enclosing the Mathias lateral ramp........................................................................................... 32

21. Strike-line seismic-reflection profile of the Highland County lateral ramp in Virginia ................................... 34

22. Part of a seismic-reflection profile of the western end of the Highland County lateral ramp in Virginia........ 36

23. SLAR images showing lateral ramps in the central Appalachians ................................................................... 38

24. Block diagram of the Seven Mountains lateral ramp showing reversal of ramp dips and null zone 

involving no displacement or hinging under Jacks Mountain, central Pennsylvania....................................... 40

25. Block diagrams showing the concept of fault spacing and fold size relative to height above 

décollement, or depth of erosion, indicated by numbered erosional levels...................................................... 40

26. SLAR image showing the junction of the central and southern Appalachians along the 

Roanoke lateral ramp ........................................................................................................................................ 41

27. SLAR composite image of the southern Appalachians showing locations of inferred lateral ramps .............. 42

28. SLAR image of parts of the Winston-Salem and Johnson City 1¡×2° quadrangles showing the 

well-defined edges of a lineament swarm......................................................................................................... 44

29. Block diagrams showing pivoting and translation of the southern Appalachians around the 

central Appalachians in the Johnson City, Tenn., area...................................................................................... 45

30. Map showing location of selected lateral ramps in the central and southern Appalachians............................. 46

31. SLAR image of the southern Appalachians...................................................................................................... 47

32. Map showing the relationship between lateral ramps and tectonic windows in the central and 

southern Appalachians...................................................................................................................................... 48

33. Block diagrams showing different geometries of thrust faults that result in the formation of 

tectonic windows............................................................................................................................................... 49
CONTENTS V

34–37. Maps showing—

34. Contoured frequency of severely thrust faulted and folded (disturbed) zones observed in the

field in the central Appalachians .................................................................................................................. 50

35. Relationship among lateral ramps, Mesozoic basins, and associated Mesozoic and Cenozoic dikes

in the central and southern Appalachians .................................................................................................... 52

36. Relationship between lateral ramps and earthquakes in the eastern United States....................................... 53

37. Relationship between lateral ramps and major N. 60°-70° W.-trending lineament zones in the

central and southern Appalachians and transform faults offshore............................................................... 54

38. Block diagrams illustrating progressive development of transform faults in new oceanic crust at

spreading center, influenced by strike-slip movement on pre-existing faults in the separating

continental margins of North America and Africa................................................................................................. 56

39. SLAR image of the western Brooks Range in Alaska........................................................................................... 58

40. SLAR image of a part of Papua New Guinea showing change in fold wavelength along which volcanoes

are aligned.............................................................................................................................................................. 60

41. Index map of China showing the approximate location of Landsat mosaic shown in figure 42 and the

two inferred lateral ramps discussed in the text..................................................................................................... 60

42. Mosaic of Landsat scenes along southern lateral ramp in China ........................................................................... 61

TABLE

 1. Proposed lateral ramps in the Appalachians and their identifying characteristics.................................................... 4

METRIC CONVERSION FACTORS

Multiply By To obtain

inch (in) 25.4 millimeter (mm) 

 foot (ft) 0.3048 meter (m)

 mile (mi) 1.609 kilometer (km)
1

Lateral Ramps in the 

Folded Appalachians and in Overthrust Belts Worldwide—

A Fundamental Element of Thrust-Belt Architecture

By Howard A. Pohn

ABSTRACT

Lateral ramps are zones where décollements change

stratigraphic level along strike; they differ from frontal

ramps, which are zones where décollements change stratigraphic level perpendicular to strike. In the Appalachian

Mountains, the surface criteria for recognizing the subsurface presence of lateral ramps include (1) an abrupt change

in wavelength or a termination of folds along strike, (2) a

conspicuous change in the frequency of mapped faults or disturbed zones (extremely disrupted duplexes) at the surface,

(3) long, straight river trends emerging onto the coastal plain

or into the Appalachian Plateaus province, (4) major geomorphic discontinuities in the trend of the Blue Ridge province, (5) interruption of Mesozoic basins by cross-strike

border faults, and (6) zones of modern and probable ancient

seismic activity. Additional features related to lateral ramps

include tectonic windows, cross-strike igneous intrusions,

areas of giant landslides, and abrupt changes in Paleozoic

sedimentation along strike.

Proprietary strike-line seismic-reflection profiles cross

three of the lateral ramps that were identified by using the

surface criteria. The profiles confirm their presence and

show their detailed nature in the subsurface.

Like frontal ramps, lateral ramps are one of two possible

consequences of fold-and-thrust-belt tectonics and are common elements in the Appalachian fold-and-thrust belt. A survey of other thrust belts in the United States and elsewhere

strongly suggests that lateral ramps at depth can be identified

by their surface effects.

Lateral ramps probably are the result of thrust sheet

motion caused by continued activation of ancient cratonic

fracture systems. Such fractures localized the transform

faults along which the continental segments adjusted during

episodes of sea-floor spreading.

INTRODUCTION

The term “lateral ramp” was used by Boyer and Elliott

(1982), Butler (1982), and Hossack (1983) to describe a

tectonic ramp that is parallel to the transport direction of

regional thrust sheets. In this report, the concept of a lateral

ramp is expanded to encompass a zone of structural disruption (faulting, folding, and possible intense fracturing)

where décollements change stratigraphic level along

structural strike.

Although discussions of individual lateral ramps in outcrop and the occasional model of a lateral ramp are not

uncommon in the geologic literature, comprehensive discussions of the detailed geometry of lateral ramps and descriptions of these ramps throughout an entire fold-and-thrust belt

do not appear to exist. Descriptions of lateral ramps in the

southern Appalachians were provided by Thomas and others

(1986) and Thomas (1990, 1991).

Because no major thrust fault (frontal ramp or décollement) can continue indefinitely along strike, the presence of

lateral ramps in fold-and-thrust belts is the result of one of

two possible inevitable occurrences. Thrust faults must either

die out by diminishing displacement (fig. 1A) or transfer their

displacement to some cross-strike fault via a lateral ramp

(fig. 1B). Close examination of structures in the field and of

geologic maps of the central and southern Appalachians

shows that large cross-strike faults are not commonly

expressed at the surface. Exceptions are rare; they include the

Jacksboro fault of the Pine Mountain thrust system (fig. 2)

and possibly the Gizzard décollement (present only as scattered klippen on the southern Appalachian Plateaus). However, proprietary seismic-reflection profiles show that largedisplacement cross-strike faults are indeed present in the subsurface and form the foundations or deflecting buttresses of

lateral ramps. The recognition of surface criteria, indicative

of these lateral ramps at depth, is fundamental to an understanding of the structural architecture of the Appalachian

fold-and-thrust belt.

Studies by the author in the Appalachian Mountains

over the last 17 years have first suggested, then confirmed, a

number of surface criteria for recognizing the locations of

subsurface or blind lateral ramps. In the Appalachian foldand-thrust belt, these criteria include (1) an abrupt change in

wavelength or a termination of folds along strike, (2) a conspicuous change in frequency of mapped faults or disturbed
2 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

zones at the surface, (3) long, straight river trends emerging

onto the coastal plain or into the Appalachian Plateaus province, (4) major geomorphic discontinuities in the trend of the

Blue Ridge province, (5) interruption of Mesozoic basins by

cross-strike border faults, and (6) zones of modern and probable ancient seismic activity. Additional features related to

lateral ramps include tectonic windows, cross-strike igneous

intrusions, areas of giant landslides, and abrupt changes in

Paleozoic sedimentation along strike (table 1).

PURPOSE AND SCOPE

The presence of subsurface frontal ramps (tectonic

ramps as defined by Harris and Milici, 1977) parallel to the

strike of the Appalachian fold-and-thrust belt has been

known since Rich (1934) first proposed such a ramp in his

model of the Pine Mountain thrust fault. Since then, others

have recognized these frontal ramps at a mesoscopic scale in

outcrops and at a megascopic scale (in both seismic-reflection sections and geologic maps at scales as small as

1:1,000,000).

Much less is either known or hypothesized about the

nature of transverse structures, although numerous researchers have discussed them or inferred their presence from lineaments on the surface. These features have been called

incipient tear faults (Rich, 1934), strike-slip fault-lineaments

(Rodgers, 1963), Gwinn-type lineaments (Kowalik and

Gold, 1974, after Gwinn, 1964), transverse faults (Harris and

Milici, 1977), cross-strike structural discontinuities (CSD’s)

(Wheeler, 1978), transverse décollements (Kulander and

Dean, 1978), and lateral ramps (Boyer and Elliott, 1982;

Butler, 1982; Hossack, 1983). Coleman (1988a) briefly

discussed the subtle differences among lineaments, CSD’s,

and lateral ramps.

In general, thrust-belt researchers assumed that these

transverse structures are probably strike-slip faults, and that

they are a result of tear faulting at the margins of thrust

sheets. This report suggests an augmentation of the strikeslip fault hypothesis as illustrated, but not discussed, by

Kowalik and Gold (1974); that is, that many of these crossstrike features may be underlain by lateral ramps that serve

to transfer décollements from one stratigraphic level to

another in the same manner as the frontal ramps, but in a

direction normal to the strike of the fold belt. This report

further explores the possibility that many of these ramps

may have considerable (mile-scale) along-strike movement

normal to, or at high angles to, the regional tectonic

transport direction.

ACKNOWLEDGMENTS

The author gratefully acknowledges the generosity of

Amoco Production Co., Houston, Tex., which provided the

seismic profiles used in this report, and of MARS Associates,

Phoenix, Ariz., which provided the radar mosaic image of

figure 40. All other radar images were produced as part of a

national survey by the U.S. Geological Survey (USGS). All

interpretations from these data are by the author.

Finally, I would like to acknowledge the late Terry W.

Offield, one of my oldest and closest friends and always my

most demanding critic. Terry read and reviewed the manuscript on two separate occasions; although I did not agree

with all his comments, the report includes most of them.

There is no doubt that the report was improved considerably

by his reviews. He will be missed as a reviewer, as a fellow

scientist, and as a friend.

Figure 1. Block diagrams of lateral ramps. Small arrows show sense of movement on fault. Full arrow indicates transport direction in

block diagrams. A, Typical “bow and arrow” structure showing diminishing displacement at the ends of a thrust fault. B, Thrust fault with

displacement lost to cross-strike faults. Top fault block is transparent.
INTRODUCTION

3

Figure 2. Side-looking airborne radar (SLAR) uncontrolled image of the southern Appalachians showing the location of the Jacksboro fault and the Pine Mountain thrust fault in

western Tennessee.
4 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

GEOMETRIC CONSIDERATIONS OF 

LATERAL RAMPS

OVERVIEW

Simplified models of the geometry of lateral ramps

show that four basic configurations are possible (fig. 3A–D).

From the simplest to the most complex, these are parallelsided ramps connected to a horizontal décollement (fig. 3A),

parallel-sided ramps connected to a rising décollement (fig.

3B), convergent-sided ramps connected to a horizontal

décollement (fig. 3C), and convergent-sided ramps

connected to a rising décollement (fig. 3D).

Each of these geometries, with the exception of the

first, produces a smaller cross-sectional area at its distal end

than at its proximal end. Thus, each of these last three

geometries requires some lateral spillover, or movement

along strike, of the compressed materials. Note that none of

these last three examples requires the presence of a frontal

ramp to initiate movement along strike. The lateral movement is produced entirely by volumetric constriction in the

direction of tectonic transport. Even the parallel-sided ramp

connected to a horizontal décollement may produce

movement along strike if the angle on the lateral ramp is

shallower than the dip of an accompanying frontal ramp or

if the materials along strike are more compressible than are

the materials along dip.

Note that the last two examples produce a paradox in

terminology. Both are more correctly considered to be

oblique ramps rather than lateral ramps at depth, but the surface manifestation of these oblique ramps shows the geometry of true lateral ramps. Conversely, a slightly different

geometry of the spillover of materials might make true lateral

ramps at depth appear to be oblique ramps at the surface.

Furthermore, the question arises as to what amount of

obliquity is permitted before a lateral ramp should be

termed an oblique ramp. If one degree of obliquity (deviation from parallelism to the transport direction) is sufficient

to change the term from lateral to oblique, then there is no

such thing as a lateral ramp. Clearly, in order to truly define

lateral and oblique ramps, one must have the benefit of

three-dimensional seismic-reflection profiles, because only

then can the worker determine the geometries of both the

surface and subsurface. In this report, because the author

does not have access to three-dimensional seismicreflection profiles, and therefore cannot determine the

geometries from subsurface to surface, all of the examples

are considered to be lateral ramps.

Table 1. Proposed lateral ramps in the Appalachians and their identifying characteristics.

[X, feature observed; N.O., change in fault frequency not yet observed; dash (—), feature not present; ?, igneous intrusion not certain]

Abrupt change Geomorphic Change in Narrowing, Seismic

 Proposed lateral in fold Plungeouts Straight river discontinuities fault frequency interruption, or profile

 ramp name wavelength of folds trends in Blue Ridge or at surface termination of evidence

equivalent Mesozoic basins

Wilkes-Barre.................... X X Susquehanna River- — N.O. — —

upper reaches of

Delaware River.

Susquehanna.................... X X Susquehanna River X X — X

Seven Mountains............. — X Middle of Juniata — X — —

River.

Tyrone-Mount Union....... X X Upper reaches of — X — —

Juniata River.

Bedford............................ X X — X X — X

Pennsylvania-Maryland-

 West Virginia................. X X Patuxent River X X X X

Mathias............................. X X Rappahannock River X X X X

Highland County.............. — X James River X X X X

Lexington......................... X X Upper reaches of X X X —

James River.

Roanoke........................... X X Roanoke River and X X X —

northwest arm of

Smith Mountain Lake.

Blacksburg....................... — X — — X — —

New River........................ — X New River X X X —

Pulaski............................. — X — — X — —

Johnson City.................... — X — X X X —

Kingsport......................... — X — X X X —

Knoxville......................... X X — X N.O. — —

Fontana Lake................... — X Tennessee River X N.O. — —

Rising Fawn*................... — X — — N.O. — —

Calhoun............................ — X — X N.O. — —

Springville........................ — X — — N.O. — —

Piedmont........................... — X — — N.O. — —

*Previously named by Thomas and Neathery (1980).
GEOMETRIC CONSIDERATIONS OF LATERAL RAMPS 5

THE BASIC MODEL

The basic model of lateral ramps assumes that two phenomena are true. The first is that structures in the Appalachians are, for the most part, scale independent (Rogers,

1858; Nickelsen, 1963; Faill, 1973; Pohn and Purdy, 1982,

1988; Pohn and Coleman, 1991). Thus, if a particular structure occurs in outcrop, it probably also occurs at other scales,

such as in thin section, at quadrangle-mapping scale, or at

seismic-reflection-profile scale. Scale independence is

important because the perception and thorough understanding of a lateral ramp at outcrop scale can yield important

clues for the detailed structural interpretation of lateral

ramps on seismic-reflection profiles.

Figure 4A is a sketch of an example of a complex ramp

anticline structure in the central Appalachian Valley and

Ridge province. The first question that comes to mind is,

“What is the scale of the example?” There are two answers

to this question. The first is that this example is a hand specimen about 5 in long. The second answer is, “it probably

doesn’t matter,” because this exposure could be 1 or 10 in

long, or 1 or 10 mi long. In fact, figure 4B shows another

sketch of a quite similar ramp anticline, but this is from a

seismic-reflection profile in the central Appalachian Valley

and Ridge province, and the anticline’s long dimension is

approximately 6 mi. Figures 4C and 4D show a photograph

of the hand specimen example and the seismic record. The

same types of features were seen in thin sections, outcrops,

and seismic-reflection profiles over at least eight orders of

magnitude (0.1 in to 10 mi) and may exist at least an order of

magnitude both smaller and larger. This qualitative

observation indicates that these geologic patterns are scale

independent and, therefore, fractal (Christopher Barton,

USGS, oral commun., 1992).

The second phenomenon, which can also be confirmed

in the field, is that faults are contemporaneous with or predate the associated folds. Field observations show that in the

Valley and Ridge and Appalachian Plateaus provinces of the

Appalachians, either anticlines in the hanging walls or synclines in the footwalls of faults are usually present. However,

the occurrence of both anticlines and synclines of the same

approximate magnitude and associated with the same fault is

rare. Seventeen years of mapping structures in the Valley

and Ridge and Appalachian Plateaus provinces has revealed

only two anticline-syncline pairs that were related to the

same fault. If the folds preceded the faults, then observations

should show an anticline in the hanging wall and a syncline

in the footwall of virtually every thrust fault, but this was not

observed. Therefore, faults must precede or be contemporaneous with their associated folds.

Table 1. Proposed lateral ramps in the Appalachians and their identifying characteristics—Continued.

 Modern Giant Abrupt change Coarse Major High

 Proposed lateral earthquakes landslides Tectonic in thickness pebble Igneous lineaments frequency Geophysical

 ramp name (1628 to (Schultz and windows of facies conglomerates intrusions crossing of mineral evidence

present) Southworth, 1989) Precambrian deposits

block

Wilkes-Barre.................... X (9) — X X X — — — —

Susquehanna..................... X (11) — X X X X — X —

Seven Mountains.............. — X — — — — — — —

Tyrone-Mount Union....... X (2) — X — — — — — X

Bedford............................ — — — X — — — — X

Pennsylvania-Maryland-

 West Virginia................. X (5) X — X X X — — —

Mathias............................ X (7) — — X — ? — X —

Highland County............. X (22) — — — — X — — X

Lexington......................... X (1) — — — — — — — —

Roanoke........................... X (5) X X X X — — — X

Blacksburg....................... X (6) X X — — — — — —

New River........................ X (7) X X X — X — — —

Pulaski............................. X (11) X X — X — — — —

Johnson City................... X (11) — X — — X — — —

Kingsport......................... X (10) X X — — — X — —

Knoxville.......................... X (17) — X — — — — — —

Fontana Lake.................... X (7) — X — — — — — —

Rising Fawn*................... X (10) — X X — X — X X

Calhoun............................ X (8) — X — — — X — —

Springville........................ X (3) — X — — — — — —

Piedmont.......................... X (7) — X — — — X — —
6 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

In addition, if folds preceded faults, then one ought to

see similar wavelengths of folds when a train of folds is

interrupted by large-displacement faults. Instead, what is

usually observed is a considerable change in wavelength of

folds as each large displacement discontinuity is crossed.

This observation also seems to indicate that faults must precede or be contemporaneous with the associated folds (Pohn

and Purdy, 1982).

From these observations and interpretations, several

inferences may be made: (1) the spacing between thrust

faults controls the wavelength of included folds; wide

spacing between thrust faults gives rise to broad folds, and

narrow spacing gives rise to small folds; and (2) as shown in

figure 5, the spacing of faults, in turn, is controlled by the

position of the fault above the décollement. Splay faults are

listric; that is, curvilinear, concave-upward surfaces to which

the master décollement is tangent. Seismic and outcrop data

show that fewer secondary and tertiary faults branch off the

lower order splay faults at depth (nearer the décollement)

than at distances farther from the décollement. Many of these

faults die out as blind thrusts (Boyer and Elliot, 1982; Butler,

1982). The convergence of higher order splay faults causes

the spacing between thrust faults nearer the décollement to be

wider than the spacing between the faults at greater distances

above the décollement (fig. 5).

Jacobeen and Kanes (1974, 1975) presented evidence

for the concept that many frontal ramps overlie areas of basement block faulting in the folded Appalachians. They

suggested that these ramps exist because of the crowding of

beds against the block fault in response to compressional

stresses resulting from Alleghanian plate convergence; these

stresses, in turn, force bedding-plane faults (décollements) to

Figure 3. Simplified block diagrams of lateral ramps showing four basic geometric configurations. Arrows show sense of movement on

fault. A, Parallel-sided lateral ramp connected to a horizontal décollement. B, Parallel-sided lateral ramp connected to a rising

décollement. C, Convergent-sided lateral ramp connected to a horizontal décollement. D, Convergent-sided lateral ramp connected to a

rising décollement.
SUPPORTING DATA 7

and shales with minor sandstones (Berg, 1980; Berg and

Dodge, 1981). Much of this stratigraphic section (fig. 12)

was penetrated by the Amoco No. 1 Wilhour Gas Unit well,

which is 14,737 ft deep and located near the crest of the

Shade Mountain anticline (fig. 11). This well yielded considerable valuable stratigraphic and structural information,

although it did not penetrate the thrust fault that appears to

core the anticline, according to the seismic-reflection profile

(fig. 13). This thrust fault lies approximately 5,380 ft below

the bottom of the well.

Shales and siltstones of the Middle Devonian Hamilton Group crop out at the well location. A normal stratigraphic section of Devonian to Lower Ordovician strata

was encountered throughout the first 11,440 ft in the well.

At this depth, the well crossed a reverse fault and repeated

the lower 1,070 ft of the section present in the hanging

wall. Dolostones of the Lower and Middle Ordovician

Beekmantown Group, and possibly the Upper Cambrian

Gatesburg Formation, are present in the footwall, where

they either are intensely folded or dip to the south at angles

of 40° to 70°.

In addition to the stratigraphic classification, the rock

assemblages in the well have been grouped into lithotectonic

units, each of which exhibits a distinctive structural geometry (fig. 12). Five of the units are described below:

Unit I. Shales of the Hamilton Group (unit I) behave as a

ductile mass and exhibit disharmonic folding, but

their shallow depth of burial reduces their contribution to the overall structural complexity. 

Unit II. The interval from the top of the Devonian Onondaga Limestone to the base of the Silurian Tonoloway Formation is unit II, which is a more

competent sequence than unit I. 

Unit III. Beneath the Tonoloway Formation is unit III, a

1,820-ft-thick (estimated true thickness) relatively

incompetent section of silty and (or) sandy shales,

including the Silurian Wills Creek Formation,

Bloomsburg Red Beds, and Rose Hill Formation,

which display generally southerly dips ranging

from 8° to 25°. 

Unit IV. Beneath the Rose Hill Formation is unit IV, which

is more competent (and more harmonically folded)

than unit III; unit IV is a sequence of medium- to

thick-bedded sandstones of the Lower Silurian

Tuscarora Formation and the uppermost part of the

Upper Ordovician Juniata Formation. 

Unit V. Underlying unit IV is unit V, which is an approximately 3,490-ft-thick interval of silty and sandy

shale and sandstone of the lower part of the Juniata

Formation and the Bald Eagle Formation and

shales of the Upper Ordovician Reedsville Shale. 

Map patterns on the geologic map of Pennsylvania

(Berg, 1980) suggest that little disharmonic folding occurs

below unit III other than in the Reedsville Shale and near the

ramp upward to a higher stratigraphic level. Figure 6 shows

dip-line seismic data illustrating just such a situation in

northeastern West Virginia. This profile supports the fault

interpretation of Jacobeen and Kanes (1974, 1975).

If basement block faults parallel to the structural strike

of the Valley and Ridge province lead to the formation of

frontal ramps, then block offsets on basement faults perpendicular or nearly perpendicular to the strike of the Valley and

Ridge province might similarly give rise to the formation of

a lateral ramp (fig. 7). If a basement block fault is present at

even a slight angle to the maximum principal stress direction

of the orogenic movement, the compressive forces cause the

rocks adjacent to basement to be refracted up the fault face,

and thus produce an environment favorable to the formation

of a lateral ramp. In addition, the steepness of the fault scarp

controls the amount of refraction of the superjacent beds.

Steep fault scarps generally refract steeply, whereas gently

dipping fault scarps refract at gentler angles (fig. 8).

FIELD EXAMPLES AND

SUPPORTING DATA

SUSQUEHANNA LATERAL RAMP

In the anthracite belt of central Pennsylvania, an obvious discontinuity exists between the wavelength of folds

generally to the east of the Susquehanna River and the wavelength of folds generally to the west of the river (fig. 9). Only

the Montour anticline (figs. 10 and 11) persists as a significant positive structural element across the river, and even this

anticline is slightly offset. The change in fold wavelength is

undoubtedly partially related to a ductility contrast in lithotectonic units (Currie and others, 1962; Nickelsen, 1963;

Wood and Bergin, 1970); however, some of the discrepancy

is related to the presence of a lateral ramp that underlies the

Susquehanna River. Supporting evidence is given below.

Solely on the basis of seismic expression, Pohn and

Coleman (1991) interpreted seismic basement as a gently

undulate surface that rises from a total depth of 47,000 ft

southwest of the river to 26,250 ft northwest of the river.

Conversely, seismic basement rises from a total depth of

42,000 ft southeast of the river to 35,000 ft northeast of the

river. This type of dip reversal indicates the presence of basement slopes in a scissors fault with the inflection (hinge)

below the Montour anticline (fig. 10). The steeper gradient to

the north and west of the river and the maximum slope on the

basement, which is at a slight angle to the tectonic transport

direction, produce an environment favorable for the formation of a lateral ramp (fig. 10).

STRATIGRAPHIC AND STRUCTURAL SETTING

The area of the Susquehanna lateral ramp is underlain

by a combination of Paleozoic carbonate rocks, siltstones,

FIELD EXAMPLES AND SUPPORTING DATA
8 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Figure 4.(above and facing page). Illustration of the scale of folds, faults, and ramps. A, Sketch of hand specimen photographed for

figure 4C, from the central Appalachian Valley and Ridge province. B, Sketch of seismic-reflection profile of figure 4D, from the Appalachian Valley and Ridge province. Arrows show sense of movement of faults. C, Photograph of the hand specimen sketched in figure

4A, from the Brallier Formation near Moorefield, W. Va. Specimen is about 5 in across. D, Seismic-reflection profile of the West Virginia Valley and Ridge province section sketched in figure 4B. Arrows show sense of movement along faults. Profile shows seismic data

collected along a line approximately 21 mi long.
SUPPORTING DATA 9 FIELD EXAMPLES AND SUPPORTING DATA

Figure 4. Continued.
10 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

basement, where the basal thrust fault produces the major

displacement in the region.

Seismic-reflection profiles show key reflectors that

are approximately coincident with the Onondaga Limestone, the carbonate rocks at the top of the Trenton

Group, and the Precambrian basement (figs. 14, 15). The

seismic expression indicates that the Precambrian

basement rises from a depth of approximately 35,000 ft in

the southeast to a depth of about 26,250 ft in the northwest. This change in elevation of approximately 8,750 ft

is almost identical to the relative elevation change of

8,950 ft expressed by the stratigraphic interval encompassing the Hamilton to Beekmantown Groups west of the

Susquehanna River.

SEISMIC EVIDENCE

Two seismic lines were used to evaluate the nature of

the lateral ramp that underlies the Susquehanna River. One

is a dip line (fig. 14), and one is a strike line (fig. 15). Three

key reflectors are present on these lines and, indeed, are

present on most seismic-reflection profiles in Pennsylvania.

These reflectors roughly correspond to the Middle Devonian

Onondaga Limestone, the carbonate rocks at the top of the

Middle Ordovician Trenton Group, and the top of

Precambrian basement.

DIP-LINE SECTION

Several significant features can be seen on the dip line

(fig. 14). Perhaps the most distinctive feature on this line is

the disharmonic folding that occurs just below the Onondaga Limestone. The décollement below the Onondaga

Limestone is located within the Silurian Wills Creek Formation. Field observations confirm that the Wills Creek

Formation gives rise to décollements in central Pennsylvania. In figure 14, the décollement appears to behead the

small anticline at the left-hand (north) side of the seismicreflection profile. This same feature appears on other

proprietary seismic-reflection profiles along strike. The

beheaded part of this anticline should appear in the hanging

wall of the décollement somewhere to the north of the section shown in figure 13; significantly, however, in proprietary lines north of and parallel to this line, no anticlinal

tops are present above the décollement. The absence

strongly implies that the top of the anticline has been transported onto the Appalachian Plateaus beyond the limit of

the available data. The distance of this transport is a minimum of 20 mi.

Figure 14 and adjacent parallel proprietary seismicreflection profiles are excellent examples of downplunge

projection (Mackin, 1950). Each anticline in the carbonate

rocks at the top of the Trenton Group can be traced westward

more than 30 mi to the folds that crop out west of the Susquehanna River. The broad Milton anticline at the level of the

Onondaga Limestone also can be traced to the surface.

In figure 14, the first syncline adjacent to the Milton

anticline at the level of the Onondaga Limestone is the southwesternmost tip of the Lackawanna syncline. In the center of

the profile, the zone that shows few or no reflections is the

Montour anticline (also known as the Berwick anticline).

The paucity of reflectors on this structure is due to limb dips

that exceed 45°. The axis and south limb of the Northumberland syncline can be seen at the right center of the profile.

The southernmost feature seen on the profile is the Shade

Mountain anticline. Note that both the Shade Mountain and

Milton anticlines are antiformal stacks as defined by Boyer

and Elliott (1982). A close examination of the smaller anticlines under the Milton anticline reveals that they, too, are

small antiformal stacks. These structures within structures

reinforce the concept that most structures in the Appalachians (and, indeed, in compressional tectonic regimes anywhere) are probably scale independent (Rogers, 1858;

Nickelsen, 1963; Faill, 1973; Pohn and Purdy, 1982; Pohn

and Coleman, 1991).

The sequence of events leading to the present structural

configuration appears to be as follows: 

1. The first structures that formed must have been the

smaller anticlines presently seen under the Milton

anticline. 

2. The tops of these anticlines were beheaded by the thrust

fault that transported the anticlinal tops out onto the

Appalachian Plateaus. No major structures existed to the

south of the smaller anticlines, because, if such structures were present, then they would show up in the hanging wall of the décollement at the level of the Wills

Creek Formation. Similarly, if the décollement formed

before the small anticlines, then the décollement would

be folded conformably with the small anticlines. 

3. Finally, the Milton, Montour, and Shade Mountain

anticlines were formed. Note that the broad arch of the

Milton anticline is mirrored by the broad arch that can be

drawn through the bases of the small anticlines subjacent

to the Milton anticline.

Figure 5. Sketch showing the relationship of a décollement to

primary, secondary, and tertiary faults. Arrows show sense of

movement on faults.
SUPPORTING DATA 11

STRIKE-LINE SECTION

A strike-line section very nearly perpendicular to the

dip-line section of figure 14 is shown in figure 15. The

uppermost major reflector at the level of the Onondaga

Limestone appears to be plunging gently to the east. Slightly

below this reflector, the décollement in the Wills Creek

Formation is weakly apparent. Still lower, an unusual

“porpoising” is apparent in the reflectors that correlate with

the top of the carbonate rocks of the Trenton Group. These

“porpoising” features are apparently anticlinal crests that

have been truncated and translated westward by a middlelevel fault below the top of the Trenton Group. Translation

of these Trenton Group crests both northward along synthetic thrust faults and westward along the same fault indicates either two nearly orthogonal directions of thrusting or

two components of a single movement. The disharmony

between the crests and the relatively flat décollement show

that the Trenton Group crests were formed first by northward-directed thrusting. As translation proceeded northward, constriction of the overlying sedimentary sequence by

a gradually westward-rising basement forced this displacement to take a westward component. Continued constriction

by the mass of the sedimentary pile and the anisotropy of the

disharmonically folded Silurian shales forced the rupture of

the otherwise coherent carbonate anticlinal crests, sheared

them off, and transported them 3 to 4 mi to the west.

Below this décollement and approximately one-third

to one-half the distance to seismic basement, two

additional faults can be seen cutting the section upward to

the west. These two lower thrust faults taken together

constitute the primary zones of dislocation along the

Susquehanna lateral ramp.

Additional information and a more thorough discussion

on the Susquehanna lateral ramp can be found in Pohn and

Coleman (1991).

PENNSYLVANIA-MARYLANDWEST VIRGINIA LATERAL RAMP

The presence of a broad lateral ramp, rising to the

south, in the region where the borders of Pennsylvania,

Maryland, and West Virginia join is strongly indicated from

field work, radar data, and proprietary seismic data. This

ramp, which strikes N. 60° W., appears to be bounded on the

north and south by the relatively straight segments of the

Potomac River southwest of Hancock, Md. (fig. 16).

North of this area, major folds in the Valley and Ridge

province are relatively broad and range from 2 to 11 mi in

width. South of the area, the folds are relatively narrow

and range from 0.25 to 5 mi in width. The surface and subsurface expressions of the ramp include more than just a

change in fold wavelength. Four lines of evidence are

described below:

1. Field mapping of disturbed zones shows that the frequency, length, and concentration of such zones

increase abruptly and conspicuously to the south of the

Pennsylvania-Maryland-West Virginia State line juncture. The disturbed zones are long, narrow zones of

intensely thrust-faulted and folded rocks intercalated in

otherwise relatively undeformed sections and represent

the surface manifestations of splay faults (Pohn and

Purdy, 1982, 1988). This increase in disturbed zones is

accompanied by an equally abrupt decrease in fold

wavelength south of the line. The increase in faulting

and decrease in fold wavelength indicate that the master

décollement is closer to the surface south of the juncture.

2. Proprietary seismic data show a cross-strike basement

extensional fault down-to-the-north accompanied by an

abrupt shallowing of reflectors to the south of the

juncture.

3. Seismic data from the Appalachian Plateaus show the

master décollement to be deeper to the north of the

Pennsylvania-Maryland-West Virginia ramp (James

Farley, petroleum consultant, oral commun., 1983).

4. The Appalachian structural front conspicuously shifts

eastward south of the Pennsylvania-Maryland-West

Virginia juncture, which would be expected if the

décollement was shallower to the south (fig. 17).

MATHIAS LATERAL RAMP

SURFACE EVIDENCE

A lateral ramp in the area of Mathias, W. Va., is suggested because of a number of fold plunges and changes

in fold wavelength across the Valley and Ridge province

and because of the generally straight course of the

Rappahannock River as it crosses the coastal plain. Field

investigations revealed an unusually high frequency of

disturbed zones as well as a small up-to-the-north lateral

ramp exposure north of Mathias.

A strike-line seismic-reflection profile (fig. 18) shows

that there is a lateral ramp in the suspected area and that the

ramp appears to rise more than 6,000 ft from north to south.

Most of this rise is lost at the southern edge of the ramp as

the lateral spillover decreases (see above section “Geometric

Considerations of Lateral Ramps” for the explanation of this

loss). An additional surface manifestation of the deep-seated

Mathias lateral ramp is reflected in the coastal plain where

the Rappahannock River trends almost straight southeastward along the northern border of the ramp.

SEISMIC EVIDENCE

The Mathias ramp is by far the most complex of the lateral ramps seen in seismic-reflection profiles. At the north

FIELD EXAMPLES AND SUPPORTING DATA
12 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE Figure 6 (above and facing page).

A, Dip-line seismic-reflection profile in northeastern West Virginia showing the relationship of tectonic ramps to basement block faults. Arrows

show sense of movement in faults. Profile shows seismic data collected along a line approximately 9 mi long. 

B, Same profile as 

A but uninterpreted. 
FIELD EXAMPLES AND SUPPORTING DATA SUPPORTING DATA 13 Figure 6. Continued. 
14 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Figure 7. Block diagram showing relationship between basement block faults and frontal ramps and between basement cross-strike faults

and lateral ramps (from Pohn and others, 1985). Arrows indicate relative movement.

Figure 8. Block diagrams showing relationship of fault refraction to steepness of fault scarp. A, Steep faults

refract steeply. B, Shallow faults refract shallowly.
SUPPORTING DATA 15

side of the section (fig. 18), the carbonate rocks of the Trenton Group are repeated. A great deal of transport appears to

be required to accomplish this doubling; however, this is a

strike line, and the transport direction is into the plane of the

figure. At the left center of the figure, the ramp reaches its

maximum height, drops in a reverse graben or triangle structure, rises again in a series of northward-directed thrusts, and

begins to lose elevation at the south edge of the profile. At

the level of the Onondaga Limestone, there are fewer faults,

but splays that rise from the arched décollement just above

the Onondaga Limestone produce considerable faulting at or

near the surface. The splay that reaches the surface at the

extreme right side of the record is exposed in a roadcut north

of the town of Mathias, W. Va. The type of deformation of

this strike-line section more closely resembles a complex dip

line and shows many of the same features that are found on

dip lines in fold-and-thrust belts. In terms of balancing cross

sections, note also that for almost any dip line at right angles

Figure 9. Side-looking airborne radar (SLAR) image of the southern part of the Williamsport 1°×2° quadrangle and the northern part

of the Harrisburg 1°×2° quadrangle, central Pennsylvania, showing broad folds of the anthracite district east of the Susquehanna River

and narrower folds west of the river. Heavy straight lines indicate boundaries of the Susquehanna lateral ramp. Width of area shown in

figure is 93 mi. See figure 10 for identification of features.

FIELD EXAMPLES AND SUPPORTING DATA
16 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

to this line, beds will have moved both in and out of the dip

lines (see Pohn and Coleman, 1991).

In a north-south roadcut, approximately 0.6 mi north

of Mathias in Hardy County, W. Va., exposures on both

sides of the road reveal an excellent example of an almost

completely exposed up-to-the-north lateral ramp (fig. 19).

This exposure in the Upper Devonian Brallier Formation

appears to be a typical disturbed zone (Pohn and Purdy,

1982, 1988) seen in dip section, with highly folded and

faulted beds both overlain and underlain by relatively

undisturbed beds. Fold vergence and slickensides show that

the transport direction of materials in this outcrop has been

in a N. 30° E. direction, nearly perpendicular to the direction of tectonic transport in the Valley and Ridge province

(Pohn and others, 1985).

Both northwestward and southeastward continuations

of the outcropping lateral ramp at Mathias are suggested on

side-looking airborne radar (SLAR) images (fig. 20A and B).

The SLAR data show a number of topographic discontinuities that are parallel to or are along the strike of the exposed

lateral ramp at Mathias (M in figure 20B).

Southeastward, the ramp area appears to be present

as sharp inflections and offsets in the ridges supported by

the Devonian Oriskany Sandstone on both sides of the

Adams Run anticline, 1.2 and 4.8 mi east of Mathias (A

and B in figure 20B). These inflections, which narrow

the anticline to the southwest, portray an up-to-the-north

ramp.

Northwestward, continuations of the across-strike

Mathias lateral ramp exposure are manifested by (1) wind

Figure 10. Block diagram showing relationship of basement faulting, change in décollement level, and change in fold wavelength to

the east and west of the Susquehanna River, central Pennsylvania. Note that the Montour anticline lies directly over the hinge line in the

basement.
SUPPORTING DATA 17

and water gaps in the Devonian Brallier, Chemung, and

Hampshire Formations from 4.5 to 7.9 mi west of Mathias

(C in figure 20B); (2) a sharp inflection and landslide in the

Elk Horn Mountain anticline 10.9 mi west of Mathias (D in

figure 20B) (C. Scott Southworth, USGS, oral commun.,

1985); and (3) a series of plunging noses (plungeouts) of

parasitic anticlines northeast and southwest of the town of

Petersburg, W. Va., 15.5 to 19.2 mi west of Mathias (E and

F in figure 20B). Although the anticlines plunge out in the

region of the Petersburg lineament of Wheeler and Sites

(1977) and Sites (1978), lines connecting the anticlinal

noses both north and south of Petersburg are much closer to

the strike of the Mathias lateral ramp (N. 60° W.) than to

the Petersburg lineament (N. 70° E.) of Sites (1978),

FIELD EXAMPLES AND SUPPORTING DATA

Figure 11. Geologic map of central Pennsylvania showing approximate locations of the seismic-reflection profiles (from Berg,

1980). Formation symbols are explained as follows: Or, Reedsville Shale; Obe, Bald Eagle Formation; Oj, Juniata Formation; St,

Tuscarora Formation; Sc, Clinton Group (predominantly Rose Hill Formation); Sbm, Bloomsburg Red Beds and Mifflintown Formation, undivided; Swc, Wills Creek Formation; DSkt, Keyser and Tonoloway Formations, undivided; Doo, Old Port Formation and

Onondaga Limestone, undivided; Dh, Hamilton Group; [PMD, Devonian, Mississippian, and Pennsylvanian formations, undivided. Solid heavy lines indicate faults. Dashed lines show the location of the Susquehanna lateral ramp. From Pohn and Coleman (1991), modified from Berg and Dodge (1981).
18 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Figure 12. Stratigraphic section penetrated by the Amoco No. 1 Wilhour Gas Unit well

(shown in figure 13) from the Shade Mountain area of central Pennsylvania. From Pohn and

Coleman (1991). Geologic series abbreviated as follows: L, Lower; M, Middle; U, Upper.

Stratigraphic thicknesses are estimates. Total depth of the well was 14,737 ft. Gp., Group.
ADDITIONAL LATERAL RAMPS IN THE CENTRAL APPALACHIANS 19

suggesting that the continuation of the structure exposed in

the roadcut north of Mathias is the dominant structure in the

region. The extensions northwestward from Mathias are

subparallel to the Lost River lineament of Sites (1978). In

addition, Evans (1989) showed a folded horse block and

hypothesized an up-to-the-north oblique ramp in the Great

Valley, to the southeast of Mathias.

HIGHLAND COUNTY LATERAL RAMP

SURFACE AND GEOPHYSICAL EVIDENCE

Rodgers (1970, p. 17–20) demonstrated that the Elkins

Valley and Browns Mountain anticlines terminate along a

line that trends N. 72° W. Southeastward along strike, this

zone is marked by a series of fold plunges in the Valley and

Ridge province east of the Elkins Valley and Browns

Mountain anticlines, as well. Progressively southeastward,

this zone coincides first with a conspicuous inflection in the

Blue Ridge Mountains in Augusta County, Va.; second,

with straight trends of the James River; third, with an

inflection in the horizontal gradient in the gravity data; and

fourth, with a break in pattern in the second vertical derivative of aeromagnetic data off the east coast of Virginia

(Krohn and Phillips, 1982).

Field investigations in the area have revealed a thrust

fault striking N. 80° E. and dipping 28° to the south, 1.25

mi north of the north end of Snowy Mountain in Pendleton

County, W. Va. This fault is the northernmost fault

connected with the Highland County lateral ramp. As in

the example of the Mathias lateral ramp, this northern border fault appears to be a smaller antithetic fault imbricate

to the major ramp whose major movement is up-to-thesouth. A strike-line seismic-reflection profile (fig. 21)

shows the subsurface nature of the Highland County lateral

ramp in Virginia.

SEISMIC EVIDENCE

Although not nearly as complex a lateral ramp as the

more deeply seated Mathias lateral ramp, the Highland

County ramp, as seen in the seismic-reflection profile (fig.

21), does show an up-to-the-south configuration. The ramp

begins halfway between basement rocks and the carbonate

rocks of the Trenton Group, displaces the carbonate rocks

slightly, and produces an anticlinal configuration that bows

up the Onondaga Limestone several thousand feet. Just

below and just above the Onondaga Limestone, faulting

increases considerably. Several faults beneath the Onondaga Limestone disappear into a zone several reflectors

below the Onondaga Limestone, and some faults rise from a

zone several reflectors above the Onondaga Limestone.

Only two small faults actually cut the limestone unit itself.

This suggests that there are décollements just below and

just above the Onondaga Limestone in the area of the

Highland County ramp.

IGNEOUS INTRUSIONS ASSOCIATED

WITH LATERAL RAMPS

Of the four lateral ramps mentioned above, three have

igneous intrusions mapped at the surface and parallel to the

proposed ramps. The Susquehanna, the PennsylvaniaMaryland-West Virginia, and the Highland County ramps

all show igneous intrusions directly over and parallel to the

lateral ramps. These are the only igneous intrusions present

at the surface in the Valley and Ridge province and west of

the Little North Mountain fault (which separates the Valley

and Ridge province from the Great Valley) in the central

Appalachians.

Although the presence of these igneous intrusions

indicates a connection to basement, no obvious basement

faulting can be seen in the seismic-reflection profile of the

Highland County ramp (fig. 21). However, an additional

seismic-reflection profile (fig. 22) does show a peculiar

signature in an otherwise featureless basement. This

anomaly lies directly below an igneous dike exposed at

the surface. The seismic-reflection profile of the Mathias

lateral ramp (fig. 18) shows a similar signature in an

almost featureless basement. If this signature is characteristic of an igneous body at depth, then the Mathias ramp

may also have been affected by igneous activity.

ADDITIONAL LATERAL RAMPS IN 

THE CENTRAL APPALACHIANS

At least five additional hypothesized lateral ramps

are present in the central Appalachians of Pennsylvania to Virginia (fig. 23A and B). They are the

Wilkes-Barre ramp, the Seven Mountains ramp, the

Tyrone-Mount Union ramp, the Bedford ramp, and the

Lexington ramp. The evidence for these ramps varies

from multiple fold plunges at Seven Mountains to fold

plunges, faulting at the surface, and reported flower

structures (Biddle and Christie-Blick, 1985) in the subsurface of the Bedford ramp. Another lateral ramp may

exist at Roanoke, Va.

WILKES-BARRE LATERAL RAMP

The Wilkes-Barre lateral ramp (fig. 23B) is defined by

a series of fold plunges across strike, which can be seen on

the SLAR images and more obviously on the geologic map

of Pennsylvania (Berg and Dodge, 1981). In addition, the

overall straight trend of the Susquehanna River from
20 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Figure 13 (above and facing page). A, Dip-line seismic-reflection profile across the Shade Mountain anticline

shown in figure 11. Faults are shown by solid lines; relative direction of movement is shown by arrows. Dashed lines

are key reflectors. Profile shows seismic data collected along a line approximately 13 mi long. From Pohn and Coleman

(1991). B, Same profile as A but uninterpreted. 
ADDITIONAL LATERAL RAMPS IN THE CENTRAL APPALACHIANS 21

Figure 13. Continued.
22 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Figure 14 (above and facing page).

A, Dip-line seismic-reflection profile across the Milton and Montour anticlines shown in figure 11. Solid lines are interpreted faults; dashed

lines are key reflectors. From Pohn and Coleman (1991). Profile shows seismic data collected along a line approximately 36 mi long. 

B, Same profile as 

A but uninterpreted.
ADDITIONAL LATERAL RAMPS IN THE CENTRAL APPALACHIANS 23 Figure 14. Continued.
24 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE Figure 15 (above and facing page).

A, Strike-line seismic-reflection profile parallel to the regional structure (fig. 11) but perpendicular to the Susquehanna lateral ramp (figs.

10, 11). Solid lines are interpreted faults; dashed lines are key reflectors. From Pohn and Coleman (1991). Profile shows seismic data collected along a line approximately 39 milong. 

B, Same profile as 

A but uninterpreted.
ADDITIONAL LATERAL RAMPS IN THE CENTRAL APPALACHIANS 25 Figure 15. Continued.
26 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Towanda, Pa., to the Lackawanna syncline and the generally

straight course of the Delaware River from Easton, Pa., to

Trenton, N.J., are directly over the proposed ramp. Eleven

occurrences of anomalous coarse pebble conglomerates have

been described by Sevon (1979) along this ramp (for more

discussion on the importance of conglomerates, see the

section below on the Fincastle Conglomerate).

SEVEN MOUNTAINS LATERAL RAMP

The Seven Mountains lateral ramp in Pennsylvania

underlies folds plunging to the west at Seven Mountains

and has folds plunging to the east at Doubling Gap, Pa.

(fig. 23A). The most probable reason for the opposite

plunges at opposite ends of the ramp is a décollement that

Figure 16. Side-looking airborne radar (SLAR) image of part of the Cumberland 1°×2° quadrangle showing the narrowing of folds to

the south across the Pennsylvania-Maryland-West Virginia lateral ramp (area within the crosscutting parallel lines).
ADDITIONAL LATERAL RAMPS IN THE CENTRAL APPALACHIANS 27

is up-to-the-west at its northern extremity and up-to-theeast at its southern extremity (fig. 24). Between these two

extremities, there is an area below Jacks Mountain where

the ramp has no displacement, which represents a null

zone. The null zone is similar to the Montour anticline

across the Susquehanna ramp, which does not exhibit any

plunge (fig. 10).

TYRONE-MOUNT UNION LATERAL RAMP

The Tyrone-Mount Union lateral ramp (fig. 23A) in

Pennsylvania is represented by abrupt changes in fold

wavelength, plungeouts of folds, and straight trends of

the Juniata River crossing the Valley and Ridge

province. Parrish and Lavin (1982), Lavin and others

(1982), and Gold and Pohn (1985) discussed the presence of geophysical discontinuities in both gravity and

magnetic data along the hypothesized ramp. As in the

example of the Seven Mountains ramp, the TyroneMount Union ramp is likely to be up-to-the-west on the

northern end and up-to-the-east on its southern end

(north of Shippensburg, Pa.).

BEDFORD LATERAL RAMP

The Bedford lateral ramp in Pennsylvania is represented by a discontinuous line of faults (Berg and Dodge,

1981), fold plunges, changes in fold wavelength, and discontinuities in the South Mountain area of the Blue

Ridge province. Proprietary seismic data show the presence of a flower structure in the subsurface (James Farley, petroleum consultant, oral commun., 1981). In

addition, a small portion of a lateral ramp can be seen on

Pennsylvania State Route 30, 328 ft to the west of its

intersection with Interstate Route 76 (Pennsylvania Turnpike).

LEXINGTON LATERAL RAMP

The Lexington lateral ramp in Virginia is manifested by

a series of fold plunges; changes in fold wavelengths;

changes in the frequency of faults mapped at the surface;

long, straight trends in the upper reaches of the James River;

discontinuities in the Blue Ridge province; and interruption

of Mesozoic basins.

ROANOKE LATERAL RAMP

There may be more surface stratigraphic and structural

information available for the area above the hypothesized

Roanoke lateral ramp than for any other lateral ramp in the

Appalachians. Unfortunately, no seismic-reflection profiles

are available and thus the subsurface aspect of the ramp is

unknown. However, the abundant stratigraphic and

structural evidence strongly suggests that there is a

Figure 17. Block diagram showing the eastward shift of the

Appalachian structural front south

of the Pennsylvania-MarylandWest Virginia lateral ramp.
28 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE Figure 18 (above and facing page).

A, Strike-line seismic-reflection profile of the Mathias lateral ramp in West Virginia. Profile shows seismic data collected along a line (fig.

20A) approximately 16 mi long. 

B, Same profile as 

A but uninterpreted.
ADDITIONAL LATERAL RAMPS IN THE CENTRAL APPALACHIANS 29 Figure 18. Continued.
30 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

significant lateral ramp at Roanoke, Va., that coincides with

the junction of the central and southern Appalachians.

A major difference between the central and southern

Appalachians is that, in the central Appalachians, the

décollements are mostly at considerable depth, whereas in

the southern Appalachians, the décollements are seen at

the surface (Rodgers, 1970, p. 39–43; Lowry and others,

1971, p. 2–6). This change in décollement level coincides with a decrease in depth to basement from the central to the southern Appalachians. The major exception to

this concept is the Pulaski décollement, which is seen at

the surface in both the central and southern Appalachians. The décollements do not suddenly come into

being at the juncture but, instead, rise steeply to the surface along a lateral ramp, which is up-to-the-south. Figure 25 shows the major imbricate faults as being

tangential to the décollement, as has been documented

from seismic data. Because the décollements are tangential to the imbricate faults, the spacing of these imbricate

faults just above the décollements is wider than the spacing at a slightly higher level above the décollements.

Concomitantly, the fold wavelength is also wider just

above the décollements than it is at a higher level. Wavelengths of folds that have become longer the higher they

are above the décollement also are longer immediately

above the décollement. This is precisely what is seen as

the juncture zone is crossed (fig. 25).

FINCASTLE CONGLOMERATE: AN INDICATOR OF THE 

ROANOKE LATERAL RAMP

The Fincastle Conglomerate of Middle Ordovician age

lies 0.6 to 1.25 mi north of the town of Fincastle, Va. (fig.

26). This deposit, described by Stow and Bierer (1937),

Butts (1940), Decker (1952), and Kellberg and Grant (1956),

is composed of coarse pebbles derived from most of the

Cambrian and Ordovician formations lower in the nearby

stratigraphic section and possibly even from the basement

complex to the southeast of the Blue Ridge. The pebbles in

the Fincastle Conglomerate are subangular to subrounded

and are composed mostly of limestone with some quartz,

vein quartz, sandstone, chert, and siltstone. Most of these

rock types would not retain the observed shapes if they were

transported very far from their origin. In addition, the deposits are geographically restricted to the middle of the proposed

ramp and do not occur outside the confines of the ramp area.

Kellberg and Grant (1956) proposed that the Fincastle Conglomerate is a submarine channel deposit whose materials

were shed from nearby highlands located to the southeast.

This idea could tie to the tectonic picture of lateral ramps proposed here if those highlands were related to the arriving

Taconian thrust sheets, which contained most of the stratigraphic units whose sediments are found in the Fincastle

Conglomerate clasts. In fact, Kellberg and Grant (1956) also

described five additional anomalous Middle Ordovician

Figure 19. Sketch of a roadcut 0.6 mi north of Mathias, W. Va., along West Virginia Route 259, showing cross section of an up-to-thenorth lateral ramp. Arrows indicate relative movement along faults. Note the “snake head” and upper boundary fault. All these features are

perpendicular to the direction of tectonic transport. This exposure is probably the northernmost thrust fault in the Mathias lateral ramp

system and is antithetic to the main system, which is up-to-the-south.
SOUTHERN APPALACHIANS 31

coarse-pebble conglomerates in the southern Appalachians.

Of the six deposits described by them, four are located

directly on the lateral ramps proposed here.

CHANGES IN UNIT THICKNESS AND LITHOLOGIES

Oliver and others (1971), Dorobek and Read (1986),

and B.A. Ferrill and W.A. Thomas (City of Huntsville, Ala.,

and University of Kentucky, respectively, written commun.,

1990) discussed or illustrated a dramatic thinning of Upper

Silurian and Lower Devonian formations at the proposed

location of the Roanoke lateral ramp and across the strike of

the fold belt. Oliver and others (1971) showed a decrease in

thickness of the total Silurian and Devonian section from

6,000 ft to 4,000 ft (northeast to southwest) and a further

decrease in thickness to 2,000 ft at the proposed New River

lateral ramp (discussed in the following section). Dorobek

and Read (1986) showed thinning of Helderberg Group

rocks from 262 ft to 131 ft at the Roanoke ramp. Oliver and

others (1971) also showed this thinning of Helderberg Group

rocks and showed a change from limestone to sandstone

(northeast to southwest) at the Roanoke ramp. B.A. Ferrill

and W.A. Thomas (see above, written commun., 1990)

showed a thinning of Devonian rocks above the Onondaga

Limestone equivalent from 6,562 ft to 1,641 ft (northeast to

southwest) at the Roanoke ramp and a thinning of Devonian

rocks below the top of the Onondaga Limestone equivalent

from 492 ft to less than 328 ft at the ramp. All these authors

presented evidence of deep water to the northeast and shallower water to the southwest starting at least in Late Silurian

time. Thus, the roots below the Roanoke lateral ramp, and

perhaps the lateral ramp itself, may have formed during the

Taconian orogeny or earlier.

COARSE-PEBBLE CONGLOMERATES 

RELATED TO OTHER LATERAL RAMPS

Sevon (1979) reported on 19 anomalous coarse-pebble

conglomerates (his polymictic diamictite) in the Spechty

Kopf and Rockwell Formations of Late Devonian and Early

Mississippian age in Pennsylvania. Of those occurrences,

16 are directly on lateral ramps as follows: 11 are located

on the Wilkes-Barre ramp, 4 are on the Susquehanna ramp,

and 1 is on the Pennsylvania-Maryland-West Virginia

ramp. Sevon (1979) stated that “the diamictites have

narrow distribution parallel to depositional strike but have

considerable distribution normal to the depositional strike.

Deposits in Carbon County in northeastern Pennsylvania

have a width of occurrence of about 3 km parallel to

depositional strike [northeast-southwest], but occur over a

distance of about 64 km normal to depositional strike

[north-south].” This pattern of deposition is similar to that

of the Fincastle Conglomerate.

Only the largest and most conspicuous lateral ramps

that cross the entire fold belt in the Appalachians are discussed in this report, but it is clear from the section on scale

independence that lateral ramps should also exist at many

scales, from inches to miles. The existence of lateral ramps

at the various scales can be documented in the field (the relatively small lateral ramp 0.6 mi north of Mathias is just one

example). The conglomerates that are not associated with

the large lateral ramps discussed in previous sections may

have been shed off of smaller lateral ramps. 

LATERAL RAMPS IN THE

SOUTHERN APPALACHIANS

If we assume that lateral ramps in the southern Appalachians manifest themselves in the same manner as those

in the central Appalachians, then a close examination of

SLAR data should enable detection of these ramps by using

the same criteria. Features such as zones of fold plunges,

changes in fold wavelength, discontinuities in the Blue

Ridge province, straight river trends, and changes in frequency of faults mapped in the field may indicate the presence of lateral ramps at depth. Radar and field data have

provided a number of candidate sites for such lateral ramps

(fig. 27A and B). The radar data also provided unanticipated information relating to the positions of lateral ramps.

Although many of the proposed ramps in the southern

Appalachians are essentially perpendicular to the local

strike of the Valley and Ridge province, several other

ramps are apparent whose strikes are close to N. 70° W., a

direction very nearly parallel to the trend of lateral ramps in

the central Appalachians. These N. 70° W.-trending ramps

are manifested not only by zones of fold plunges and

changes in fold wavelength in the Valley and Ridge

province, but by swarms of lineaments or long, relatively

uninterrupted lineaments in the Blue Ridge and in a few

places in the Piedmont provinces.

Perhaps the best example of such a lineament zone is

seen on the radar image of the Winston-Salem and Johnson

City 1°×2° quadrangles (fig. 28). On this image, the lineament zone is seen not only within the main body of the Blue

Ridge province, but it appears to cross the northward extension of the Brevard fault zone as well. This lineament zone

(herein named the Kingsport lateral ramp) strikes N. 73° W.

and crosses the Precambrian rocks of the Blue Ridge

province but is apparently only weakly expressed in the

Paleozoic rocks of the Valley and Ridge province. The local

cross-strike lateral ramp (herein named the Johnson City

lateral ramp), which strikes N. 17° W., crosses both the

Valley and Ridge and Blue Ridge provinces. These two

ramps intersect at Johnson City, Tenn. One ramp is parallel

to ramps in the central Appalachians and one is perpendicular to the local strike of the southern Appalachians. The

LATERAL RAMPS IN THE SOUTHERN APPALACHIANS
32 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE
SOUTHERN APPALACHIANS 33

Figure 20 (previous page and above). Side-looking airborne radar (SLAR) images of parts of the Cumberland and Charlottesville

1°×2° quadrangles showing the boundary lines enclosing the Mathias lateral ramp. East-west line across image is a mosaic line. A,

Image showing approximate location of the strike-line seismic-reflection profile in figure 18 (shown by ticked line). B, Inset from

right-center of A. Heavy arrows mark the Mathias lateral ramp. Key features are as follows: A and B, offsets of ridges underlain by

Oriskany Sandstone, illustrating the topographic discontinuities along the northern hinge of the ramp; C, wind and water gaps; D, inflection and landslide in Elk Mountain anticline; E and F, termination of subsidiary anticlines. M, Mathias, W. Va. Scene is approximately 34 mi wide. From Pohn and others (1985).

LATERAL RAMPS IN THE SOUTHERN APPALACHIANS
34 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE Figure 21 (above and facing page).

A, Strike-line seismic-reflection profile of the Highland County lateral ramp in Virginia. Profile shows seismic data collected along a line

approximately 12 mi long. 

B, Same profile as 

A but uninterpreted.
SOUTHERN APPALACHIANS 35 Figure 21. Continued.

LATERAL RAMPS IN THE SOUTHERN APPALACHIANS
36 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Figure 22. Part of a seismic-reflection profile of the western end of the Highland County lateral ramp in Virginia. Profile shows seismic data collected along a line approximately

17 mi long.
TECTONIC WINDOWS 37

Kingsport ramp (which trends N. 73° W.) is most conspicuous in the Blue Ridge and Piedmont rocks and is only weakly

expressed in the lower Paleozoic Valley and Ridge rocks;

this indicates that the Kingsport ramp may have formed first

and that the southern Appalachians, including the Johnson

City ramp, pivoted around the Kingsport ramp (fig. 29). In

fact, the aeromagnetic map of the United States (Zietz, 1982)

shows that the trend of the central Appalachian Blue Ridge

province continues into the southern Appalachians. The

converse is unlikely to be true because the Kingsport ramp

has virtually the same strike as other central Appalachian

ramps, whereas the Johnson City ramp is perpendicular to

the local strike of the fold belt but is not parallel to the majority of other ramps. The lineament swarm in the Blue Ridge

province along the Kingsport ramp may be a still earlier zone

of deep basement fracturing that, through reactivation, is

manifested in the tectonically superposed and rotated Precambrian and Paleozoic covers. Two distinct directions of

probable ramps exist not only here but in at least two other

places in the southern Appalachians.

The proposed New River and Blacksburg lateral ramps

show the same relationship as the Johnson City and

Kingsport ramps. The New River ramp (N. 17° W.) is perpendicular to the local strike of the southern Appalachians.

The Blacksburg ramp strikes N. 65° W. Both ramps exhibit

fold plunges in the Valley and Ridge province and disruptions in the Blue Ridge province. The trend of the New River

in the Valley and Ridge and Appalachian Plateaus provinces

is coincident with the New River ramp (hence its name).

B.A. Ferrill and W.A. Thomas (City of Huntsville,

Ala., and University of Kentucky, respectively, written

commun., 1990) showed an abrupt decrease in thickness of

Devonian rocks above the rocks equivalent to the Onondaga Limestone from 3,609 ft east of the New River to

1,641 ft west of the ramp. They also showed an abrupt

facies transition from sandstone east of the ramp to shale

west of the ramp. Joseph O’Connor (USGS, oral commun.,

1990) found an abrupt change across the New River gorge

in the sandstones of Pennsylvanian age. To the northeast of

the gorge, the sandstones have more rounded grains and

have little feldspar present. To the southwest of the gorge,

the sandstones have more angular grains, contain more

feldspar, and have well-preserved mica flakes. Both of

these observations are consistent with the presence of a

subaqueous, down-to-the-northeast slope in Devonian

through Pennsylvanian time. This slope would have been

appropriately positioned to influence the formation of the

lateral ramp.

The proposed Knoxville lateral ramp (perpendicular to

the local strike) and the Fontana Lake lateral ramp (N. 69°

W.) likewise show the same relationships as the two previously illustrated intersecting ramp pairs.

Other lateral ramps with their proposed names are presented in figure 30. The Rising Fawn CSD (cross-strike

structural discontinuity), here called the Rising Fawn lateral

ramp, was originally named by Thomas and Neathery

(1980). Coleman (1988a,b) presented a very thorough discussion of the manifestations of the Rising Fawn CSD (or

lateral ramp), including sedimentation, fold plunges, mineral

deposits, and modern seismicity.

Two proposed lateral ramps interpreted from the SLAR

data neither strike N. 70° W. nor are perpendicular to the

local strike of the fold belt. They are here named the

Calhoun and Piedmont lateral ramps. The Calhoun ramp

connects a series of fold plunges and a 70° change in the

local strike of the Valley and Ridge province and projects

east-northeastward to the Warwoman lineament (Hatcher,

1974). The Piedmont ramp, farther to the south, connects a

pair of apparent left-lateral shear zones (fig. 27, this report).

LINEAMENT SWARMS OF EASTERN 

TENNESSEE AND WESTERN NORTH 

CAROLINA

A swarm of long, throughgoing lineaments can be

seen on the SLAR images from eastern Tennessee and

western North Carolina (fig. 31). These lineaments range

from 31 to more than 217 mi in length. All strike N. 62° to

74° W., with the exception of the lineament that strikes N.

47° W. and passes through Englewood, Tenn. Although

these lineaments may not be related to lateral ramps, the

coincidence of their strikes with the approximate N. 70° W.

direction and their persistence across country without

regard to topography or geology make them likely candidates for the usually deeply buried root zones of lateral

ramps. Occasional folds plunge out against those lineaments although not in the numbers seen along other ramps

described in this report.

LATERAL RAMPS AND

TECTONIC WINDOWS

Tectonic windows are eroded areas of thrust sheets that

display the rocks beneath the thrust sheet (Gary and others,

1972). Although the distribution of windows appears to be

random, there is a marked coincidence of lateral ramps and

tectonic windows, such as the Birmingham window with the

Tyrone-Mount Union lateral ramp in Pennsylvania. Inspection of available information suggests that, of the 55 tectonic

windows in the central and southern Appalachians, 53 are

directly coincident with proposed lateral ramps (fig. 32).

The remaining two are along an indistinct, but visible, lineament that can be traced from the Appalachian structural front

through the Blue Ridge province. Although no folds plunge

out along this lineament, the strike of the lineament is N. 67°

W.; therefore, the lineament may be an additional root zone

of a lateral ramp.

LATERAL RAMPS AND TECTONIC WINDOWS
38 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Figure 23 (above and facing page). Side-looking airborne radar (SLAR) image showing lateral ramps in the central Appalachians. A, Overview. Ramps shown by lines are abbreviated as

follows: SQ, Susquehanna; SM, Seven Mountains; T, TyroneMount Union; B, Bedford (shown by single line because it is probably a single scissors fault, but may be a ramp with ill-defined

edges); P, Pennsylvania-Maryland-West Virginia; M, Mathias;

H, Highland County; L, Lexington; R, Roanoke. B, Enlarged

portion of SLAR mosaic off east edge of A showing the WilkesBarre ramp; see figure 30 for the position of the Wilkes-Barre

ramp east of the Susquehanna ramp.
TECTONIC WINDOWS 39

Figure 23. Continued.

LATERAL RAMPS AND TECTONIC WINDOWS
40 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Figure 24. Block diagram of the

Seven Mountains lateral ramp showing

reversal of ramp dips and null zone

involving no displacement or hinging

under Jacks Mountain, central Pennsylvania.

Figure 25. Block diagrams showing the concept of

fault spacing and fold size relative to height above

décollement, or depth of erosion, indicated by

numbered erosional levels. A, Erosional surface 1 is

highest above décollement where folds and faults are

few and widely spaced, as in the Valley and Ridge

province in central Pennsylvania. B, Erosional surface

2 is deeper, or closer to the décollement where folds

and faults are more numerous and closely spaced, as

in the Valley and Ridge province in Maryland, West

Virginia, and Virginia north of the Roanoke lateral

ramp. C, Erosional surface 3 is closest to the

décollement where folds and faults are fewer and

more widely spaced, as in the Valley and Ridge province in Virginia south of the Roanoke lateral ramp.
TECTONIC WINDOWS 41 Figure 26. Side-looking airborne radar (SLAR) image showing the junction of the central and southern Appalachians along the Roanoke lateral ramp (between parallel lines).

LATERAL RAMPS AND TECTONIC WINDOWS
42 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Figure 27 (above and facing page).

A, Side-looking airborne radar (SLAR) composite image of the southern Appalachians showing locations of inferred lateral ramps. Abbreviations

are as follows: 

N, New River; 

B, Blacksburg; J, Johnson City; 

P, Pulaski; K, Kingsport; KV, Knoxville; S, Sequatchie Valley; 

R, Rising Fawn; 

G, Gadsden; 

C, Calhoun; PM, Piedmont;

A, Anniston; BH, north edge of Birmingham ramp. 

B, Same image as 

A but uninterpreted.
TECTONIC WINDOWS 43 Figure 27. Continued.

LATERAL RAMPS AND TECTONIC WINDOWS
44 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Figure 28. Side-looking airborne radar (SLAR) image of parts of the Winston-Salem and Johnson City 1°

×2° quadrangles showing the well-defined edges of a lineament swarm

striking N. 73° W. and projecting northwestward to plunging fold noses in the Valley and Ridge province. Lateral ramps are abbreviated as follows: J, Johnson City lateral ramp; K,

Kingsport lateral ramp.
TECTONIC WINDOWS 45

If a tectonic window is present and is the result of a simple eroded thrust fault or duplex structure, then the window

manifests itself by a symmetrical, highly elongated geometry. The along-strike dimension is proportional to the adjacent folds (fig. 33A). Similarly, if the folds within the duplex

structure porpoise slightly, then the windows are symmetrical and have swellings and constrictions along strike (fig.

33B).

Conversely, if the window is skewed to the strike of the

adjacent folds (fig. 33C), or if the window is blunt at one or

both ends or is highly asymmetrical (fig. 33D), then the

geometry demands the presence of a lateral ramp beneath the

thrust or duplex structure (fig. 33C and D). The steepness of

the ramp will determine the degree of asymmetry. The

structural architecture for the formation of a tectonic window

appears to require both a lateral ramp and its intersection

with a décollement or its frontal ramp.

From north to south along the Appalachians, as basement becomes shallower and décollements appear at the surface southwest of the Roanoke reentrant, the first major

concentration of tectonic windows occurs at the surface. In

Tennessee, where numerous décollements are at the surface

and lateral ramps are common, tectonic windows also are

common. Significantly, in the north-central Appalachians

and in the most southerly part of the southern Appalachians

where the master décollements are deep, few tectonic windows are present. An examination of figure 18 suggests a

probable explanation of this. Structures identical to those

that make tectonic windows can be seen in this seismic

profile, but the level of erosion would have to be thousands

of feet deeper in order to exhume these structures and create

these windows.

The three largest tectonic windows (Mountain City,

Grandfather Mountain, and Sauratown on figure 32) appear

to be considerably larger than the associated hypothesized

ramps. A possible explanation is that the windows are partly

controlled by lateral ramps that underlie the Blue Ridge as

well as by the narrower lateral ramps discussed in this report.

The characteristic asymmetry of the windows is consistent

with lateral ramp geometry.

Figure 29. Block diagrams showing pivoting and translation of

the southern Appalachians around the central Appalachians in the

Johnson City, Tenn., area. Arrows show direction of relative movement of upper block. A, Strike-slip-faulted basement with future décollement surface shown by heavy dashed line. B, Rotation of

basement block on décollement surface. C, Translation of the basement and cover sequence. Plunging folds developed over the lateral

ramp that is, in turn, formed over strike-slip faults. Note that erosion

of the cover sequence high above the décollement will leave folded

cover rocks in the Valley and Ridge province, but farther south and

east in the Blue Ridge or Piedmont province, erosion closer to the

décollement will expose the root zone of the basement. The

basement strike-slip faults will then appear as lineaments, as near

Kingsport, Tenn.

LATERAL RAMPS AND TECTONIC WINDOWS
46 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

LATERAL RAMPS AND FREQUENCY 

OF DISTURBED ZONES

Disturbed zones (fig. 34) are sequences of severely

thrust faulted and folded rocks either in the hanging walls or

footwalls adjacent to thrust faults or between closely spaced

pairs of thrust faults (Pohn and Purdy, 1982, 1988; Pohn and

others, 1985). Most areas where there are numerous

disturbed zones lie either along the margins of, or directly

over, lateral ramps. Repeated stick-slip movement over the

lateral ramps during the Alleghanian orogeny probably

caused an increase in faulting associated with the lateral

ramps. The highest frequency of disturbed zones (40) is

directly over the Roanoke lateral ramp, probably one of the

most highly displaced and definitely the most rotated of any

of the lateral ramps in the Appalachians.

Figure 30. Map showing location of selected lateral ramps in the central and southern Appalachians. Bedford ramp and extensions of

other ramps shown by light dashed lines. Two lateral ramps in the northeast corner of the map are in the northern Appalachians and are

unnamed. Note the lineament swarms, represented by single heavy dashed lines, that more or less parallel some ramp lines in the

southern half.
LATERAL RAMPS AND FREQUENCY OF DISTURBED ZONES 47 Figure 31. Side-looking airborne radar (SLAR) image of the southern Appalachians. Arrows indicate lineament swarm in western North Carolina and eastern Tennessee. This is the same image shown in figure 2.
48 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Figure 32. Map showing the relationship between lateral ramps, lineaments, and tectonic windows in the central and southern Appalachians. See figure 30 for ramp names. Named windows are listed above. MO, unnamed window encompassed by part of Mauchono fault

of Wood and Bergin (1970). Unlabeled windows are unnamed in the literature. Toxaway and Tallulah domes are shown by dashed lines to

reflect uncertainty as to whether or not they are true windows.
LATERAL RAMPS AND POSSIBLE BASEMENT FAULTING 49

LATERAL RAMPS AND POSSIBLE 

BASEMENT FAULTING

With the exception of the Bedford lateral ramp, no

direct evidence connects lateral ramps to faults in the underlying basement; there is, however, some inferential evidence

for many of the proposed ramps. Table 1 shows that six

ramps have swarms of igneous intrusions above the ramps;

the Mathias ramp may also have swarms of intrusions above

it. Moreover, these swarms are parallel to the lateral ramps. 

Table 1 shows that all but three proposed ramps (Seven

Mountains, Bedford, and Lexington) have a high frequency

of modern earthquakes coincident with them (see the following section). In fact, the Bedford ramp shows the presence of

a flower structure in the basement directly beneath the ramps

(James Farley, petroleum consultant, oral commun., 1985).

This evidence, coupled with the fact that there are mapped

faults at the surface along the whole of the Bedford ramp,

indicates that the Bedford ramp probably has a direct connection to basement. This does not imply a direct connection

between basement and the superjacent lateral ramp in all

cases but does imply that movement in the basement (probably strike-slip) caused differential compression without

accompanying cross-strike failure in the cover rocks. This

differential movement in the cover rocks, accompanied by a

differential cumulative thickness on either side of the lateral

ramp, is responsible for the conspicuous change in wavelength of folds along strike.

Unfortunately, seismic-reflection profiles, the most

important source of evidence, do not show any obvious

Figure 33. Block diagrams showing different geometries of thrust faults that result in the formation of tectonic windows. Teeth are on

upper plate of thrust fault; small arrows show fault movement sense. A, Tectonic window formed by erosion of a simple duplex. B, Tectonic

window formed by erosion of a slightly warped or undulating duplex. C, Skewed tectonic window exposed by erosion near the intersection

of a lateral ramp and a frontal ramp. D, Blunt-ended tectonic window formed from a pair of lateral ramps.
50 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

faults in the basement. This lack of data is partly due to the

fact that most hydrocarbon exploration companies generally

do not run strike-line seismic profiles. Even if there were

many more strike-line profiles, basement strike-slip faults

would be unlikely to show unless there was a significant

velocity contrast across the fault. In addition, if the faults

were active with pure strike-slip motion, then there would

be little or no topographic relief on the fault. Coleman

(1988b) illustrated seismic evidence for basement offset

beneath the Anniston lateral ramp and cited nonseismic

geophysical evidence for basement offset beneath other

similar features.

MOVEMENT ALONG LATERAL 

RAMPS THROUGH TIME

Proprietary seismic-reflection data show that alongstrike basement faults were active as growth faults from

Precambrian until at least Middle Ordovician time. Crossstrike faults also were at least intermittently active for this

same period of time. Activity on the cross-strike faults

does not appear to have continued past Middle Ordovician

time. A map showing the relationship between Mesozoic

basins and lateral ramps in the central and southern Appalachians (fig. 35) shows that horst blocks of Precambrian

rocks crossing the basins, narrowing of the basins, and eastwest border faults all occur over eastward extensions of

many of the proposed lateral ramps. In addition, Mesozoic

intrusions are coincident with at least six of the lateral

ramps. Continued reactivation until the present is strongly

indicated by the presence of Eocene intrusions along the

Highland County lateral ramp and by the observation that

more than 48.7 percent of modern earthquakes since 1628

(Earth Technology Corporation, written commun., 1984)

are directly coincident with lateral ramp positions (fig. 36).

This is in spite of the fact that lateral ramps occupy no more

than 15 percent of the geographic area in the central and

southern Appalachians.

Figure 34 (above and facing page). Map showing contoured frequency of severely thrust faulted and folded (disturbed) zones observed

in the field, per 7.5-minute quadrangle in relation to inferred lateral ramps in the central Appalachians. Contour interval is 5 disturbed zones.

Hachures indicate closed area of fewer disturbed zones. Note peak frequency of 40 disturbed zones centered over the Roanoke lateral ramp.

Single dashed line in southwestern Virginia marks major lineament parallel to ramps; there is not enough information to define both sides

of the suspected ramp.
POSSIBLE RELATIONSHIP OF LATERAL RAMPS TO OFFSHORE TRANSFORM FAULTS 51

OBSERVED ASSOCIATION OF 

LATERAL RAMPS AND GIANT 

LANDSLIDES

Schultz (1986) and Schultz and Southworth (1989)

have reported on giant, ancient landslides in the central

and southern Appalachians. Some of these landslides are

mile-sized blocks that are tens to hundreds of feet thick

and probably are the largest landslide blocks east of the

Great Plains. Each of these landslides either is directly on

a lateral ramp or is between two closely spaced lateral

ramps. Although the landslides have no direct connection

to the lateral ramps, the high frequency of earthquakes

coincident with the lateral ramps suggests that earthquakes may have acted as triggering mechanisms for the

landslides (see the sections above “Lateral Ramps and

Possible Basement Faulting” and “Movement Along

Lateral Ramps Through Time”).

POSSIBLE RELATIONSHIP OF 

LATERAL RAMPS TO OFFSHORE 

TRANSFORM FAULTS

A map of lateral ramps in the central and southern

Appalachians shows that nearly two-thirds of the postulated

ramps have strikes of N. 60° to 70° W. This direction is the

same as the strike of most of the transform faults mapped

offshore (Schouten and Klitgord, 1977) (fig. 37A). In fact,

extending the lateral ramp zones offshore produces an

alignment of two ramps with transform fault zones. Perhaps

more significant is the observation that the spacing of the

continental ramps and the spacing of the oceanic transform

faults are very similar, but the two appear to be offset left laterally 25 mi from one another. Removal of that offset brings

the two sets into direct alignment in both strike and spacing

(fig. 37B). This “coincidence” appears to be too fortuitous

to be truly coincidental. This suspected major strike-slip

movement, along with similar strike-slip movement on

Figure 34. Continued.
52 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

EXPLANATION

 

Lateral ramp—Dashed where ramp extends beyond

 SLAR coverage or beneath coastal plain

Lineament on SLAR image or lateral ramp of

 undetermined width

Mesozoic basin

Mesozoic or Cenozoic dikes

Lake

Erie

80 73° ° 43°

79°

32°

88°

N.Y.

Pa.

N.J.

Ohio

W. Va.

Ky.

Va.

N.C.

Tenn.

Ala. Ga.

S.C.

Md.

Del.

Mass.

Conn.

Vt.

Atlantic Ocean

0 50 100 MILES

major continental structures like the Brevard fault zone,

probably occurred as the Alleghanian orogeny ceased.

A reactivated fundamental fracture system extended

offshore as the North American and African continents separated during episodes of sea-floor spreading to produce

transform faults in the new oceanic crust. This same fracture

system was, during earlier times of compression, responsible

for the formation of lateral ramps in the overlying cover

rocks (fig. 38).

THE PRESENCE OF 

LATERAL RAMPS WORLDWIDE

Examination of Landsat and SLAR images of major

fold-and-thrust belts shows that, of the three main lateralramp recognition criteria, the two that can be identified

photogeologically (narrowing or plunging out of fold noses

across an entire fold-and-thrust belt and long, straight river

Figure 35. Map showing relationship among lateral ramps, Mesozoic basins, and associated Mesozoic and Cenozoic dikes in the central

and southern Appalachians. See figure 30 for ramp names.
LATERAL RAMPS WORLDWIDE 53

segments crossing a fold belt) are present in almost every

instance. Probable lateral ramps were identified on SLAR

and Landsat images of the following regions: the Ouachita Mountains of Arkansas; the Rocky Mountains;

Morocco, south of the Anti-Atlas Mountains; western

Yugoslavia in the Dinaric Alps; the Brooks Range of

Alaska, where they are associated with tectonic windows;

Papua New Guinea; China; and numerous other fold-andthrust belts. In addition, J.L. Coleman, Jr. (Amoco

Production Company, oral commun., 1990), has seen both

lateral ramps and associated adjacent tectonic windows at

Semail Gap in Oman. Three of these examples are

presented in more detail below.

A review of publicly available data from fold-andthrust belts around the world reveals that surface indicators

of lateral ramps are present in every belt examined to date.

The surface expressions of these lateral ramps are easily discernible on images from both spacecraft and aircraft. When

other geophysical data, such as seismic-reflection profiles,

become available, they probably will reveal additional

information on the detailed nature of lateral ramps—a

fundamental aspect of fold-and-thrust-belt architecture.

Figure 36. Map showing relationship between lateral ramps and earthquakes in the eastern United States. See figure 30 for ramp names.

Earthquake data are from Earth Technology Corporation (written commun., 1984). Modified from Pohn and Coleman (1991).

THE PRESENCE OF LATERAL RAMPS WORLDWIDE
54 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

LATERAL RAMPS IN THE

BROOKS RANGE, ALASKA

The Brooks Range crosses the entire width of Alaska

from approximately lat 68° to lat 70° N. Like the Appalachians, the Brooks Range is a fold-and-thrust belt where

folds are conspicuous because of the presence of erosionally resistant units. However, unlike the Valley and Ridge

province where both synclines and anticlines are apparent,

in the Brooks Range foothills, only the synclines are

conspicuous. This circumstance is similar to that of the

Appalachian Plateaus province; in fact, the Brooks Range

foothills appear to represent a tectonic regime similar to

that of the Appalachian Plateaus.

SLAR images of the western Brooks Range were

obtained by the USGS in 1980. The radar images, which

include both north-looking and south-looking directions, are

mosaics of the Point Hope, De Long Mountains, Misheguk

Mountain, Howard Pass, Point Lay, Utukok River, Lookout

Ridge, and Ikpikpuk River 1°×2° quadrangles (fig. 39A).

Figure 37 (above and facing page). A, Map showing relationship between lateral ramps and major N. 60°–70° W.-trending lineament

zones in the central and southern Appalachians and transform faults offshore (transform faults modified from Schouten and Klitgord, 1977).

The transform faults are expressed as gaps in the positive magnetic sea-floor spreading lineations. Only two positive magnetic sea-floorspreading lineation zones (nos. 15 and 25; Schouten and Klitgord, 1977) are represented. B, Same as A but with 25 mi left-lateral movement.

Because of the left-lateral movement, latitude and longitude are not shown.
LATERAL RAMPS WORLDWIDE 55

The most conspicuous features on the images are zones in

which folds change wavelength or orientation or plunge out

along their strikes. These zones persist from the northern

foothills through the Brooks Range itself, although in the

more mountainous areas, the zones of plunging folds are

partly replaced by long, straight river segments. By analogy

to the Appalachian fold belt, these zones probably represent

areas above lateral ramps, where décollements change stratigraphic level along strike.

Approximately 5,000 mi of unmigrated, commondepth-point seismic-reflection lines were acquired between

1974 and 1980 by Tetra Tech, Inc., under contract for the

National Petroleum Reserve in Alaska (NPRA) (Wilcox

and others, 1988). Seismic stations were spaced

approximately 0.25 mi apart, and the quality varied from

very good in the northern Arctic foothills and coastal plain

to very poor in the southern Arctic foothills. Several of the

available compressed seismic-reflection profiles in the

Brooks Range confirm the presence of lateral ramps.

The westernmost of the hypothesized lateral ramp

zones differs from those located to the east in that it shows

not only fold plunges and straight river segments, but also

abrupt changes in the strike of fold axes and conspicuous

fold interference patterns. This type of interference pattern

occurs in at least two other geographic areas as seen in

Landsat and SLAR images: in China and south of the AntiAtlas Mountains of northwestern Africa. Structures in these

areas probably are the result of plate collisions. A collision

from the west may have contributed to the formation of the

other lateral ramps in the Brooks Range.

THE PRESENCE OF LATERAL RAMPS WORLDWIDE

Figure 37. Continued.
56 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Figure 38. Block diagrams illustrating progressive development of transform faults in new oceanic

crust at spreading center, influenced by strike-slip

movement on pre-existing faults in the separating

continental margins of North America and Africa.

These continental basement faults are inferred to be

associated with lateral ramps in the cover rocks.

Cover rocks are transparent in block diagrams. A,

Initial upwelling of oceanic crust and offset along

continental faults precursory to formation of transform faults. B, Sea-floor spreading, upwarping of

continental crust, and additional movement along

continental and incipient transform faults. C, Additional spreading and fault movement.
CONCLUSIONS 57

Each of the easternmost three zones (fig. 39B) exhibits

changes in fold plunge, narrowing of folds along strike, or

long, straight stream segments nearly perpendicular to the

tectonic transport direction. The exception is a narrow zone

at approximately lat 68°45 ́ N., where folds show no apparent plunge along strike. This band probably represents a

“null” zone similar to a zone crossing the Susquehanna lateral ramp in Pennsylvania (Montour anticline, fig. 10); the

same type of zone is found on the Seven Mountains ramp in

Pennsylvania (fig. 24). Fold plunges to the north and south

of the null zone should trend in opposite directions.

Seismic-reflection profiles and geologic maps of the area

north of the null zone in the Brooks Range of Alaska (John

S. Kelley, USGS, written commun., 1991) suggest that each

of the easternmost hypothesized lateral ramps is up-to-theeast; geologic maps of the area south of the null zone also

suggest that the ramps are up-to-the-west in the main part

of the De Long Mountains.

LATERAL RAMPS IN PAPUA NEW GUINEA

Figure 40 shows a proprietary SLAR image of Papua

New Guinea near the intersection of the Papuan and Aure

thrust belts. This image shows a conspicuous change in

fold wavelength along strike. Coincident with this change

in fold wavelength is a change in depth to basement from

30,000 ft in the east to less than 16,500 ft in the west

(Ronald H. Gelnett, MARS Associates, oral commun.,

1987; Davies, 1990). This abrupt change in fold wavelength is probably due to a lateral ramp at depth, although

the presence of a ramp cannot be confirmed because no

seismic-reflection profiles are available. As seen on the

SLAR image, a line of volcanoes that occurs along this

zone of abrupt change in depth to basement and a change

in fold wavelength strongly suggest that the lateral ramp is

underlain by a basement fracture system. Additional lateral ramps within the Papua New Guinea fold belts are

illustrated in the radar images of Dekker and others (1990,

figs. 11 and 12) and aerial photographs of Dow (1977, figs.

16 and 29).

LATERAL RAMPS IN CHINA

In the late 1970’s, the USGS produced an unpublished

uncontrolled Landsat mosaic of China from individual

image chips (Frank Sidlauskas, USGS, oral commun.,

1989). Unfortunately, the scale of the mosaic is too small

to show the features discussed in this report. This mosaic

shows two discrete zones in China where every fold that

approaches the zone either changes width or, more commonly, plunges out (figs. 41 and 42). The northern zone is

more than 1,367 mi in length, and the southern zone is

more than 1,119 mi in length. Both zones are absolutely

straight at the scale of the mosaicked image. Figure 42

shows a pair of individual Landsat scenes along a part of

the southern zone that displays typical fold plunges indicative of lateral ramps.

CONCLUSIONS

1. Lateral ramps are a natural part of the architecture of

fold-and-thrust belts.

2. The positions of lateral ramps are indicated on SLAR and

Landsat images by abrupt changes in fold wavelength,

zones of plunging fold noses, and (or) long, straight river

systems all across a fold belt.

3. Evidence for lateral ramps on seismic-reflection profiles

may be as simple as a single along-strike fault, which

rises gradually through the stratigraphic column; more

commonly, the lateral ramps are multiply faulted and

folded zones whose seismic signatures resemble complex

dip lines.

4. Although uncommon, lateral ramps in outcrop resemble

complex disturbed zones or duplexes.

5. Igneous intrusions parallel to the ramps are not uncommon and may indicate a connection to basement faults.

6. Lateral ramp zones may be associated with rapid changes

in stratigraphic thickness or lithology.

7. The root zones of highly eroded lateral ramps appear to

manifest themselves as lineament swarms, as seen on

SLAR and Landsat images.

8. The formation of virtually all tectonic windows appears

to require the presence of a lateral ramp and its intersection with a décollement or frontal ramp.

9. Disturbed zones reach a peak frequency at the margins of

lateral ramps.

10. Lateral ramps appear to be connected to a Precambrian

fault system that was reactivated throughout much of

geologic time.

11. Seismic events common under lateral ramp zones may

serve to trigger giant landslides in the Appalachians.

12. A reactivated fundamental fracture system extended

offshore as the continents separated during episodes of

sea-floor spreading and influenced the inception of

transform faults in the new oceanic crust. This same

fracture system was responsible, during earlier times of

compression, for the formation of lateral ramps in the

overlying cover rocks.

13. Lateral ramps are present in fold-and-thrust belts

worldwide.
58 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE Figure 39 (above and facing page).

A, Side-looking airborne radar (SLAR) image of the western Brooks Range in Alaska. 

B, Same image as A, showing positions of hypothesized

lateral ramps.
CONCLUSIONS 59 Figure 39. Continued.
60 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

Figure 40. Side-looking airborne radar (SLAR) image of a part of Papua New Guinea showing change in fold wavelength along which volcanoes are aligned. Inferred lateral ramp is marked by arrows. Illumination is from south. Image

courtesy of MARS Associates (Phoenix, Ariz.).

Figure 41. Index map of China showing the approximate location

of Landsat mosaic shown in figure 42 and the two inferred lateral

ramps discussed in the text. 
CONCLUSIONS 61 Figure 42. Mosaic of Landsat scenes E-1525-02475-7-01 and 1488-02424-7-01, along southern lateral ramp (indicated by parallel lines) in China (see fig. 41). Image shows an area approximately 201 mi wide.
62 LATERAL RAMPS IN THE FOLDED APPALACHIANS AND IN OVERTHRUST BELTS WORLDWIDE

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shales in south-central Pennsylvania and Maryland, 44th

Annual Field Conference of Pennsylvania Geologists, Bedford, Pa., Oct. 5–6, 1979: [Harrisburg, Pa., Field Conference of

Pennsylvania Geologists] p. 61–66.

Sites, R.S., 1978, Structural analysis of the Petersburg lineament,

central Appalachians: Morgantown, W. Va., West Virginia

University, unpublished Ph.D. dissertation, 434 p.

Stow, M.H., and Bierer, J.C., 1937, The significance of an Athens

conglomerate near Fincastle, Virginia [abs.]: Virginia Academy of Science Proceedings, 1936–37, p. 71.

Thomas, W.A., 1990, Controls on locations of transverse zones in

thrust belts: Eclogae Geologicae Helvetiae, v. 83, no. 3, p.

727–744.

———1991, The Appalachian-Ouachita rifted margin of southeastern North America: Geological Society of America

Bulletin, v. 103, no. 3, p. 415–431.

Thomas, W.A., and Neathery, T.L., 1980, Tectonic framework of

the Appalachian orogen in Alabama, in Frey, R.W., ed., Excursions in southeastern geology, v. 2: Falls Church, Va., American Geological Institute, p. 465–526.

Thomas, W.A., Neathery, T.L., Ferrill, B.A., and Osborne, W.E.,

1986, Appalachian thrust belt, Alabama: American Association of Petroleum Geologists, Field Trip Guidebook, 107 p.

Wheeler, R.L., 1978, Cross-strike structural discontinuities; possible exploration tool in detached forelands [abs.]: Geological

Society of America Abstracts with Programs, v. 10, no. 4, p.

201.

Wheeler, R.L., and Sites, R.S., 1977, Field studies of Parsons and

Petersburg structural lineaments, West Virginia [abs.]: American Association of Petroleum Geologists Bulletin, v. 61, no. 5,

p. 840–841.

Wilcox, L.A., Coit, T.A., and Magoon, L.B., 1988, The National

Petroleum Reserve in Alaska computer data and graphics system; an aid to geologic interpretation, in Gryc, George, ed.,

Geology and exploration of the National Petroleum Reserve in

Alaska, 1974 to 1982: U.S. Geological Survey Professional

Paper 1399, p. 923–940.

Wood, G.H., Jr., and Bergin, M.J., 1970, The Valley and Ridge and

Appalachian Plateau; structure and tectonics; structural controls of the anthracite region, Pennsylvania, in Fisher, G.W.,

Pettijohn, F.J., Reed, J.C., Jr., and Weaver, K.N., eds., Studies

of Appalachian geology, central and southern: New York,

Interscience Publishers, p. 147–160.

Zietz, Isidore, compiler, 1982, Composite magnetic anomaly map

of the United States, pt. A, Conterminous United States: U.S.

Geological Survey Geophysical Investigations Map

GP–954–A, scale 1:2,500,000.
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